1. Field of the Invention
The present invention relates to a configuration of an image scanning apparatus suitably used to place and scan a large amount of documents.
2. Description of the Related Art
For example, Japanese patent Laid-Open application publication No. 2006-16093 discloses a copying machine which includes an upper limit position detection sensor for detecting a topmost document while being able to accommodate plural documents on a document tray (document feeder tray). In the copying machine disclosed in Japanese patent Laid-Open application publication No. 2006-16093, when the number of documents is decreased as the documents on the document tray are fed, a control device controls a tray driving mechanism to always keep the topmost document at the same position with respect to a pickup roller according to a detection result of the upper limit position detection sensor. Therefore, the topmost document is always located at a position where the document can be fed.
In the large capacity image scanning apparatus as disclosed in Japanese patent Laid-Open application publication No. 2006-16093, with the increasing need for fast copying of a large amount of documents, there is a strong demand to perform high speed scanning while maintaining the scanning image quality.
However, in Japanese patent Laid-Open application publication No. 2006-16093, there is no description concerning the timing for controlling the tray driving mechanism to lift and lower the document tray. Accordingly, in Japanese patent Laid-Open application publication No. 2006-16093, when the document tray is elevated during scanning of the document, the document is possibly moved by an elevation of the document tray during transportation of the document, resulting in deterioration of the scanning image quality.
In the configuration disclosed in Japanese Patent Laid-Open application publication No. 2006-16093, when a scanning speed of the document is increased, vibration becomes significant in conveying the document, which results in a considerable variation in the detection result of the upper limit position detection sensor. Accordingly, the top most document is often not positioned with high accuracy to frequently generate a document transfer error.